The present invention relates to a digital transmission system in which audio data and content data, such as video data and the like other than the audio data, are transmitted from a transmitting side to a receiving side and then processed on the receiving side, and a device for reproducing an audio clock, which device is provided on the receiving side of the digital transmission system.
A standard referred to as a DVI (Digital Video Interface) standard has been considered as a standard for transmitting a video signal from a signal source, such as a video disk reproducing device, a video tape reproducing device, or a personal computer, as digital video data.
In this DVI standard, a video signal is transmitted as data digitized in pixel units for each of RGB (red, green, and blue) color signals through a DVI cable (a cable specified in the DVI standard). Since the video data is digitized in pixel units, it is possible to transmit high-quality images.
Since this DVI standard concerns the transmission of video data, however, when audio data is transmitted simultaneously with video data, the audio data needs to be transmitted by a transmitting means other than the DVI cable. This complicates the configuration of the transmission system.
Accordingly, WO 02/078336 (PCT/JP02/02824) proposes a method of multiplexing audio data and video data for transmission as follows.
Specifically, the method of WO 02/078336 superimposes audio data in a horizontal blanking period or a vertical blanking period of video data, and transmits the video data and the audio data. This makes it possible to transmit video data and audio data simultaneously in an existing video data transmission format such as the DVI standard and by one transmitting means such as the DVI cable.
However, while this method directly transmits a pixel clock (reference clock) for video data as a clock for data processing, it does not directly transmit a clock for audio data, that is, an audio clock, and instead transmits information representing a frequency dividing ratio between the pixel clock and the audio clock. On the receiving side, the audio clock is reproduced by a PLL (Phase Locked Loop) circuit on the basis of the information of the frequency dividing ratio and the pixel clock.
Specifically, as shown in FIG. 6, the frequency fp of the pixel clock is 27 MHz, for example. The frequency fa of the audio clock corresponds to an audio sampling frequency fs. For example, when fs=48 kHz, the frequency fa of the audio clock is 18.432 MHz, which is 384 times 48 kHz. When fs=44.1 kHz, the frequency fa of the audio clock is 16.9344 MHz, which is 384 times 44.1 kHz. The audio clock frequency fa is expressed asfa=384fs=(N/M)fp  (1)Thus, instead of the audio clock itself or information representing the audio sampling frequency fs, information representing the frequency dividing ratios M and N is transmitted.
As shown in FIG. 6, the frequency dividing ratios M and N are M=27000 and N=18432 when the audio sampling frequency fs is 48 kHz, and the frequency dividing ratios M and N are M=30000 and N=18816 when the audio sampling frequency fs is 44.1 kHz.
On the receiving side, the audio clock is reproduced by an audio PLL as shown in FIG. 7 on the basis of the information of the frequency dividing ratios M and N and the pixel clock transmitted from the transmitting side.
Specifically, this audio PLL 60 has a VCO (Voltage Controlled Oscillator) 61. A frequency divider 71 divides the frequency fp=27 MHz of the pixel clock by M, and thereby provides a reference signal having a frequency fr=fp/M. A frequency divider 72 divides the frequency of an output clock of the VCO 61 by N. Letting fo be the frequency of the output clock of the VCO 61, the frequency divider 72 thereby provides a comparison signal having a frequency fc=fo/N. A phase comparator 73 compares phases of the reference signal and the comparison signal with each other. An error signal representing a result of the comparison is supplied to a loop filter 74. Then, an output voltage of the loop filter 74 is supplied as a control voltage Vctl to the VCO 61 to control the oscillation frequency of the VCO 61, that is, the frequency fo of the output clock.
Specifically, when M=27000 and N=18432, the frequency fc of the comparison signal from the frequency divider 72 becomes 1 kHz, which is equal to the frequency fr of the reference signal from the frequency divider 71. The VCO 61 is controlled so that the oscillation frequency fo of the VCO 61 is 18.432 MHz. Thus, an audio clock of 18.432 MHz is obtained as the output clock of the VCO 61. When M=30000 and N=18816, the frequency fc of the comparison signal from the frequency divider 72 becomes 900 Hz, which is equal to the frequency fr of the reference signal from the frequency divider 71. The VCO 61 is controlled so that the oscillation frequency fo of the VCO 61 is 16.9344 MHz. Thus, an audio clock of 16.9344 MHz is obtained as the output clock of the VCO 61. The audio PLL 60 is thus configured.
However, it is difficult to deal with the two audio clock frequencies of 18.432 MHz and 16.9344 MHz by one oscillation frequency range of one VCO 61 in the audio PLL 60 as shown in FIG. 7. In practice, it is necessary to provide a VCO having an oscillation frequency range for 18.432 MHz and a VCO having an oscillation frequency range for 16.9344 MHz, and change between the two VCOs in correspondence with the frequency of an audio clock to be reproduced.
However, in a standard referred to as an HDMI (High Definition Multimedia Interface) standard for the digital transmission system described above, information representing the audio sampling frequency fs cannot be superimposed on audio data and transmitted as information for locking the audio clock reproducing PLL on the receiving side.
It is therefore conceivable for the information representing the audio sampling frequency fs to be transmitted from the transmitting side to the receiving side by a transmitting means other than a DVI cable for transmitting video data and audio data, and for the VCO oscillation frequency range to be changed on the receiving side on the basis of the information.
This, however, requires not only the other transmitting means but also an encoder and a decoder on the transmitting side and the receiving side for the information representing the audio sampling frequency fs, thus complicating the configuration of the transmission system.
Furthermore, a temporal shift occurs due to a difference in transmission time between the information representing the audio sampling frequency fs and the pixel clock and the information of the frequency dividing ratios M and N transmitted together with the video data and audio data. A problem therefore occurs in that even when the signal source of the audio signal is changed on the transmitting side and thus the audio sampling frequency fs and the frequency dividing ratios M and N are changed, the VCO oscillation frequency range cannot be changed immediately on the receiving side. Another problem occurs when the information representing the audio sampling frequency fs does not correspond to the audio data being transmitted because of an encoding error on the transmitting side or the like and thus the audio clock corresponding to the audio data transmitted cannot be obtained.